INITIAL SUPPLY

INITIAL SUPPLY

BY WILLIAM F. ECKMAN

The amount of water available for the initial water supply is not as important as when and how it becomes available. There is a window of opportunity between the time of ignition and flashover when a fire can be extinguished with a timely offensive attack before much damage is done. If this window of opportunity is missed, the fire may accelerate to the point where it will be impossible to control with the available resources. The fire department may have to go into a defensive mode of operation and contain the fire until enough of the available fuel has been consumed, reducing the size of the fire to a point at which it can be extinguished with the water supply available.

TANK SIZE

The 1991 issue of NFPA 1901, Standard for Pumper Fire Apparatus, requires that the water tank have a capacity of at least 500 gallons. Many fire departments specify larger tanks when they purchase a new pumper. Pumpers designed for use in rural and suburban areas frequently are equipped with 1,000-gallon water tanks. Pumper tankers may carry as much as 2,000 gallons of water as well as a full load of hose and standard pumper equipment. Other departments have chosen to limit the size of the water tank on the attack pumpers and include a tanker on each structural alarm to provide the required initial water supply.

In NFPA 1231, Water Supplies for Suburban and Rural Fire Fighting, the National Fire Protection Association recommends that a minimum of 3,000 gallons be provided for any structure fire, with additional water required for large buildings and special risks. The initial water supply required for any given fire department or location depends on the needed fire flow and the length of time that will be involved in getting a sustained water supply established.

NEEDED FIRE FLOW

Both NFPA 1231 and the Insurance Services Office (ISO) recommend that the initial engine company be able to apply a minimum fire flow of 500 gpm to single-family residences within a few minutes of arrival. Multifamily residential structures, commercial or industrial buildings, and exposure protection may require a higher flow.

In an article published in the September 1992 issue of Fire Engineering, author C. Bruce Edwards pointed out that a fire can be darkened quickly with a minimum amount of water by applying enough gallons per minute to exceed the critical flow rate. The critical flow rate occurs when the water being applied to a fire is enough to absorb the amount of heat being generated, but no more. If the application rate is below the critical flow rate, it will have very little effect on the fire or at least delay the extinguishment. If the application rate is 50 percent higher than the critical flow rate, the fire will darken in less than five seconds.

The author uses an example of a large, fully involved 2,000-square-foot fire. If the initial engine company attempts to attack a fire of this magnitude with handlines, even a 212-inch handline, it is not likely to be successful. On the other hand, if the fire is attacked with a 1,000-gpm master stream, it can be controlled in only a few seconds. Even a pumper equipped with a 500-gallon tank would have enough water to supply 1,000 gpm for as long as 30 seconds.

AVAILABLE FIRE FLOW

Even though the initial apparatus may bring enough water to the scene, it may not be able to apply it to a fire. When the initial response is limited to one or two engine companies, there is not enough personnel to put into service the handlines needed to exceed the critical flow rate. A master-stream device mounted on the pumper, preconnected to the pump`s discharge, will enable a limited number of firefighters to apply as much as 1,000 gpm to a fire if the pump and the line to the devices are capable of supplying it.

The minimum required tank-to-pump flow rate specified in NFPA 1901 is 500 gpm. A maximum flow rate of about 600 gpm from the tank is typical of the fire department pumpers being constructed at the present time. Older pumpers may have maximum flow rates less than 500 gpm. Unless the fire department specified a higher flow rate when the apparatus was built, it may not be able to supply the flow needed to use a master-stream device effectively from the tank.

TANKER SUPPLY

Most fire department tankers are designed for use in a water shuttle rather than for initial supply. NFPA 1901 and 1903 both require apparatus with a water tank with a capacity larger than 1,000 gallons to transfer water to an external outlet at a rate of at least 1,000 gpm. Many manufacturers meet this requirement by supplying a gravity quick dump or jet dump for rapid unloading. Either method works well for water-shuttle operations, but both require a portable tank, and setting it up causes a certain amount of delay. Using a tanker to satisfy the initial water-supply requirements generally in-volves pumping the water to the attack pumper. For this type of operation, the tanker would have to be equipped with a 1,000-gpm pump that can supply the needed fire flow.

INITIAL WATER SUPPLY

As a general rule, fire departments protecting a suburban or rural area should bring a minimum of 2,000 gallons of water when they arrive at the incident scene. This will be enough to operate two initial attack lines and a 212-inch backup handline for at least four minutes. The attack pumper should also be able to apply this water to a fire immediately on arrival at a rate of 1,000 gpm through a preconnected master-stream device without connecting external supply lines. It should be able to maintain this rate of flow until at least 90 percent of the water has been discharged.

The initial attack and water supply apparatus should be constructed in such a way that the transition to an external source can be made without interrupting the flow to the attack lines as soon as a sustained water supply is available.

Part 3 will discuss various methods of establishing and maintaining a sustained water supply. n

WILLIAM F. ECKMAN spent 25 years in the volunteer fire service and served as a chief officer in Pennsylvania and Maryland. He is a fire protection consultant specializing in department evaluation and master planning with an emphasis on water supply. He has conducted numerous seminars and specialized training programs involving fire pumps and fire department water supply and authored Fire Department Water Supply Handbook (Fire Engineering Books, 1994).

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